Image pickup apparatus (video cameras) comprising an imaging signal generating section adapted to generate imaging output signals corresponding to the image of a subject obtained by way of an optical system that contains a solid-state imaging element and acquire video signals representing the image of the subject according to the imaging output signals have been and being popularly marketed. The solid-state imaging element contained in the imaging signal generating section arranged in such an image pickup apparatus includes light receiving parts, each of which performs an operation of photoelectric conversion upon receiving light from the subject by way of the optical system and accumulates the signal charges obtained as a result of the operation, a charge sending out section for sending out the signal charges accumulated in the light receiving part and an output part for generating imaging output signals according to the signal charge sent out from the charge sending out section.
The charge sending out section of a solid-state imaging element may be formed by using a charge coupled device (CCD) adapted to sequentially transfer the signal charges accumulated in the light receiving parts and read out from the light receiving parts, a complementary metal oxide semiconductor (CMOS) adapted to send out the signal charges accumulated in the light receiving parts according to a switching operation or some other device. An image pickup apparatus comprising a solid-state imaging element having a charge sending out section formed by using a CCD, or a so-called CCD image sensor, is normally referred to as CCD camera, whereas an image pickup apparatus comprising a solid-state imaging element having a charge sending out section formed by using a CMOS, or so-called CMOS image sensor, is normally referred to as CMOS camera.
Meanwhile, when an image is picked up by an image pickup apparatus under a light source that periodically flickers such as a fluorescent lamp, the picked up image shows periodical light and dark stripes to consequently give rise to a phenomenon of flowing light and dark stripes in the image. Otherwise, the entire image becomes periodically light and dark from frame to frame. This is a phenomenon called flicker, which constitutes an inevitable problem when an image is picked up by an image pickup apparatus comprising an image sensor adapted to pick up an image with shifted timings of accumulating electric charges under a flickering light source.
Conventionally, an image sensor shifts timings of accumulating electric charges on a frame by frame basis or on a line by line basis. Generally, a system of matching the timing of electric charge accumulation on a frame by frame basis is referred to as global shutter system, whereas a system of matching the timing of electric charge accumulation on a line by line basis is referred to as rolling shutter system. While CCD image sensors provided with a global shutter system have been in the main stream, CMOS image sensors are attracting attention because they consume less electric power and can be manufactured with a reduced number of parts at low cost. CMOS image sensors are more often than not provided with a rolling shutter system from the structural viewpoint. With either of the systems, flicker appears due to the difference of timing of electric charge accumulation when an image is picked up under a light source that flickers, although the mode of flicker may differ between a frame flicker where an entire frame become light or dark from frame to frame and a line flicker where an image flickers on a line by line basis.
FIG. 1 illustrates how the amount of the electric charge accumulated in a sensor changes when the global shutter system is employed and FIG. 2 illustrates an image that flickers from frame to frame when the global shutter system is employed, while FIG. 3 illustrates how the amount of the electric charge accumulated in a sensor changes when the rolling shutter system is employed and FIG. 4 illustrates an image that flickers on line by line basis.
For instance, when picking up an image by means of a CMOS camera and the light source that illuminates the subject flickers with a period that corresponds to the period of the AC power supply, e.g. a period equal to ½ of the period of the AC power supply, such as a fluorescent lamp, the video signals formed on the basis of the imaging output signals from the solid-state imaging element can contain a flicker component that appears as periodic rises and falls of the luminance level. Such a flicker component contained in video signals is produced as the amount of the electric charges accumulated in the light receiving parts of the solid-state imaging element, which accumulate signal charges for a predetermined period and sends out the accumulated signal charges in each predetermined period, periodically changes under the influence of the periodic fluctuations of the luminance of the light source illuminating the subject.
When the luminance of the light source illuminating a subject periodically fluctuates as indicated by X in FIG. 3 while a signal charge is accumulated in light receiving part Ln, which is a row of light receiving elements for accumulating a signal charge corresponding to a line period of a video signal in a solid-state imaging element section S, in a period (a+b) extending over period a and period b, and then a signal charge is accumulated in light receiving part Ln+1, which is a row of light receiving elements for accumulating a signal charge corresponding to the next line period of a video signal in the solid-state imaging element section S, in a period (b+c) extending over period b and period c (c=a), the amount of the signal charge accumulated in the light receiving part Ln in the period a+b is proportional to the sum of area Aa and area Ab (area Aa+Ab) and the amount of the signal charge accumulated in the light receiving part Ln+1 in the period b+c is proportional to the sum of area Ab and area Ac (area Ab+Ac). Each of the period a and the period c corresponds to a line period of the video signal and each of the period a+b and the period b+c corresponds to a frame period of the video signal.
Thus, the light receiving part Ln and the light receiving part Ln+1 show the same duration for an electric charge accumulation time (exposure time) but the timing of the electric charge accumulation time of the latter part is shifted from the timing of the electric charge accumulation time of the former part by a period corresponding to a line period of the video signal. In the case of FIG. 3, the period of fluctuations of the luminance of the light source illuminating the subject (½ of the period of the AC power supply) and the frame period of the video signal do not show a relationship that one is integer times of the other.
As clearly seen from FIG. 3, the area Aa+Ab and the area Ab+Ac commonly include the area Ab and hence the difference between them is equal to the difference between the area Aa and the area Ac. Therefore, the amount of the signal charge accumulated in the light receiving part Ln in the period a+b and the amount of the signal charge accumulated in the light receiving part Ln+1 in the period b+c show a difference that corresponds to the difference between the area Aa and the area Ac.
In this way, the amount of the signal charge that corresponds to a line period of the video signal and is accumulated in the light receiving part, which is a row of light receiving elements for accumulating a signal charge corresponding to a ling period and sending it out in the solid-state imaging element section S, periodically changes according to the periodic fluctuations of the luminance of the light source illuminating the subject. Then, the periodic change in the amount of the signal charge that is accumulated in the light receiving part and corresponds to a line period of the video signal appears as periodic fluctuations of the luminance level of the video signal to make the video signal contain a flicker component.
The image of a frame period that is reproduced according to the video signal that contains such a flicker component typically shows light and dark stripes running along the lines (in the horizontal direction) in FIG. 4.
Under these circumstances, several proposals have been made to date for flicker correction of reducing the clicker components contained in the video signal obtained from an image pickup apparatus comprising a solid-state imaging element when the image pickup apparatus is operated to pick up an image of a subject illuminated by a fluorescent lamp.
One of such proposals is known to those skilled in the art. According to the proposal, the flicker component contained in a video signal is reduced by making an electric charge accumulation period, which corresponds to a frame period of the video signal, of each light receiving part of the solid-state imaging element section for accumulating a signal charge corresponding to a line period of the video signal and sending it out equal to integer times of the flicker period of the fluorescent lamp so as to make the amount of the signal charge accumulated in a light receiving part substantially always show a constant level.
According to another proposal as described in Jpn. Pat. Appln. Laid-Open Publication No. 2000-004382, when the field rate (twice of the frame rate) of the video signal is 60 Hz and the field period is 1/60 sec while the power supply frequency of the fluorescent lamp is 50 Hz and the flicker period of the fluorescent lamp is 1/100 sec so that the fluorescent lamp flickers five times in every three field periods of the video signal and hence a same flicking pattern is repeated with a period equal to the three field periods of the video signal, the video signal is accumulated in a memory means by an amount corresponding to the number of field periods that by turn corresponds to the period of repetition of the flicker component or the integrated value thereof and the flicker component contained in the video signal is approximated by a sinusoidal wave signal. Then, the flicker component contained in the video signal is reduced by utilizing the characteristics of the sinusoidal wave signal and the video signal accumulated in the memory means or the integrated value thereof.